1. Field of the Invention
The present invention relates to a tunable dispersion compensator and a method of manufacturing the tunable dispersion compensator, which are used to compensate for the dispersion of an optical signal in an ultra high-speed optical communication system. The tunable dispersion compensator is used to control a temperature distribution of a fiber grating formed in a part of an optical fiber for transmitting the optical signal or a temperature distribution of a chirped grating whose refractive index period changes in the longitudinal direction of an optical waveguide, thereby controlling a group delay time characteristic of the grating.
2. Description of the Related Art
In an ultra high-speed optical communication system whose transmission rate is equal to or higher than 10 Gb/s, the waveform of an optical signal is distorted by the chromatic dispersion of an optical fiber serving as a transmission line. Therefore, a dispersion compensator for compensating for the chromatic dispersion of the optical fiber is required. In particular, in an ultra high-speed optical communication system whose transmission rate is equal to or higher than 40 Gb/s, a spectral width of the optical signal is 4 times that in the system of 10 Gb/s and a time per bit is ¼ that in the system of 10 Gb/s. As a result, the influence of the dispersion on the system of 40 Gb/s becomes 16 times that on the system of 10 Gb/s, so that high-precision dispersion compensation is necessary. Thus, a tunable dispersion compensator capable of tunably controlling the dispersion of the optical fiber to dynamically compensate for the dispersion thereof is a device essential for the optical communication system whose transmission rate is 40 Gb/s.
An example of the device for dynamically compensating for the dispersion of the optical fiber includes a tunable dispersion compensator in which a plurality of heater elements are provided along a fiber grating and the power of each of the heater elements is adjusted to control a temperature distribution of the fiber grating, thereby changing the dispersion of the fiber grating. The tunable dispersion compensator is disclosed in JP 2003-195234 A (lines 45 to 48 in left section of page 5, lines 6 to 10 in right section of page 5, and FIG. 3). In this type of the tunable dispersion compensator, the plurality of heater elements is linearly formed on a surface of a quartz substrate along the axis of the fiber grating. Conditions required for the heater elements are that an area of each of the heater elements is small, the number of heater elements is large, a heater element interval is short, and the like. In particular, the heater element interval is important to make the temperature distribution of the fiber grating linear.
Heater elements identical to the above-mentioned heater elements are disclosed in Matsumoto, et. al, “Tunable Dispersion Equalizer with a Divided Thin-Film Heater for 43-Gb/s RZ Transmissions”, IEEE Photon. Technol., Lett., Vol. 13, No. 8, pp. 827 to 829, August, 2001 (lower right section of p. 827 and FIG. 1). According to this description, the number of heater elements is 32 in a case of a grating having a length of 40 mm. Each of the heater elements has a width of 60 μm and a length of 1245 μm. A heater element interval is 5 μm.
In recent years, studies for bringing the optical communication system whose transmission rate is 40 Gb/s into a metro network have been popular. The metro network, which is also called a metropolitan area network (MAN), is a network for an area corresponding to a city level. In this network, when a path is blocked by network failure, it is necessary to switch from the path to another path using an optical switch or the like to maintain communications. A problem which occurs at this time is a change in length of an optical fiber transmission line which is caused by path switching. That is, a 1.3 μm zero-dispersion single-mode optical fiber which is normally used has a chromatic dispersion of approximately 17 ps/nm/km at a wavelength of 1550 nm. An allowable dispersion in a case where the transmission rate of 40 Gb/s is approximately ±100 ps/nm. Therefore, when the change in length which is caused by the path switching becomes 5.9 km or more, the waveform of the optical signal is distorted by the chromatic dispersion of the optical fiber, with the result that sufficient transmission quality cannot be ensured.
In order to solve such the problem, more specifically, in order to compensate for the chromatic dispersion caused by the path switching, the use of a tunable dispersion compensator is studied. It is desirable to complete the recovery of the path from the network failure in a shortest possible time. Therefore, a tunable dispersion compensator having a shortest possible dispersion change time is required.
In order to shorten the dispersion change time of the tunable dispersion compensator on this requirement, the following tunable dispersion compensator is described in, for example, JP 2004-258462 A (lines 17 to 47 in page 4, lines 15 to 35 in page 6, and FIGS. 1, 4, and 5). In this tunable dispersion compensator, a plurality of thin film heaters, each of which is made of tantalum nitride, are formed on a surface of a quartz substrate having a thickness of 0.1 mm. A chirped fiber grating is provided on the thin film heaters formed on the quartz substrate. A heat spreader and a Peltier device are provided on a rear surface of the quartz substrate. Power of each of the thin film heaters is controlled to apply a predetermined temperature gradient to the chirped fiber grating, thereby controlling the dispersion. With respect to the tunable dispersion compensator, a response time of the dispersion change and power consumption are measured in a case where the thickness of the quartz substrate is set to 0.02 mm, 0.1 mm, 0.5 mm, or 1 mm and the temperature gradient applied to the chirped fiber grating is set to +50° C. to −50° C. or −50° C. to +50° C. As a result, the response time of the dispersion change shortens as the thickness of the quartz substrate is reduced, thereby increasing the power consumption.
In the conventional tunable dispersion compensators, it is required that the number of heater elements is large. However, when the number of heater elements increases, there is a problem in that the number of connection wirings for the respective heater elements becomes larger and thus it is difficult to mount the heater elements. The heater element interval is a small interval of 5 μm, so there is a problem in that an advanced manufacturing technique is required and it is difficult to reduce a cost of the substrate (heater substrate) on which the heater elements are formed.
As described above, the response time of the dispersion change shortens as the thickness of the quartz substrate is reduced. However, much of heat applied to the thin film heaters flows to the heat spreader side, so that there is a problem in that the power consumption becomes larger. When the thickness of the quartz substrate is to be minimized to shorten the response time, the heat applied to the thin film heaters is immediately diffused to the heat spreader. Therefore, there is a problem in that the predetermined temperature gradient of 50° C. cannot be applied to the grating.